Fix unchecked lower bound warning in get_default_error_string.

[PDeets]

This resolves #248 by fixing a build warning on MSVC that occurs in the get_default_error_string function. The warning is due to n being used as an index without its lower bound being checked. To fix this, I'm adding a cast to the bounds comparison so it is done as unsigned. That way negative values will show up as large positive values that exceed error_unknown. For consistency with the surrounding code, I used the same syntax to define the unsigned type as is used in the is_extended function on line 45.